Re: [Xen-devel] [PATCH 01/24] xen/mm: Don't use _{g, m}fn for defining INVALID_{G, M}FN

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.06.17 at 18:20, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/06/17 17:13, Julien Grall wrote:
>> INVALID_{G,M}FN are defined using static inline helpers _{g,m}fn.
>> This means, they cannot be used to initialize a build time static variable:
>>
>> In file included from mm.c:24:0:
>> xen/xen/include/xen/mm.h:59:26: error: initializer element is not constant
>>  #define INVALID_MFN      _mfn(~0UL)
>>
>> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
>> ---
>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
>> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> Cc: Tim Deegan <tim@xxxxxxx>
>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
>>
>> I know that this solution will not work for non-debug build. I would
>> like input from the community on way to fix it nicely.
> 
> Hmm - a proper typedef gets inserted.  I presume the compiler objects to
> (unsigned long){ ~0UL } to initialise a scalar?
> 
> It might be better to move the definition of INVALID_$FOO into the
> TYPE_SAFE() declaration so we can create an appropriate initialiser for
> each both builds.

Except that you can't put a #define in a macro definition, and producing
a static const wouldn't help Julien's case of wanting it in the initializer of
a static variable. So best I can come up with right now would be to
introduce a separate

#define TYPE_SAFE_CONSTANT(name, val) \
    (name##_t){ val }

and

#define TYPE_SAFE_CONSTANT(name, val) \
    (name##_t)(val)

for the !NDEBUG / NDEBUG cases respectively and use it as

#define INVALID_MFN TYPE_SAFE_CONSTANT(mfn, ~0UL)

Jan


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